Recently-developed liquid crystal displays generally include a liquid crystal panel and a backlight unit. A liquid crystal panel of the liquid crystal displays includes a pair of transparent supporting substrates retaining liquid crystals therebetween, data lines and scan lines formed on one supporting substrate, a common electrode formed on another supporting substrate, and polarizers arranged on planes of incidence and projection, respectively, of the liquid crystal panel.
The data lines and scan lines are provided to divide display regions of the liquid crystal panel into a plurality of pixels, each pixel being provided with a pixel electrode, together with a semiconductor apparatus such as a thin film transistor (TFT) or the like. A drain region of the TFT is connected to the pixel electrode, a source region thereof is to the data lines, and a gate is to the scan lines.
The TFT performs a switching operation according to signals from the scan lines, and through which electric current flows between the data lines and the pixel electrode. Consequently, an electric field is produced between the pixel electrode and the common electrode, and enables liquid crystal molecules on the pixel electrode to change the array thereof.
Light from the backlight unit is incident upon the liquid crystal panel through the polarizer on the side of the plane of incidence. Here, light rays in the same polarization direction as a polarizing plane of the polarizer are incident upon the liquid crystal panel. TFTs of the respective pixels are controlled according to image data to be displayed, together with the polarized state of liquid crystals in the respective pixels. Incident rays of light are polarized depending upon the polarized state of liquid crystals and are incident upon the polarizer on the side of the plane of projection. Only the light rays having the same polarization direction as that of the polarizing plane are projected, so that image data are displayed as optical density of luminance.
The TFT is formed by implanting impurities on patterned polycrystalline silicon on a supporting substrate such as glass or the like so as to form a source region (electrode) or a drain region (electrode), and then performing an annealing process to activate the impurities.
Since a semiconductor such as polycrystalline silicon generates optical excitation due to incident light, if light from the backlight unit is incident upon the TFT, leakage photocurrent is generated by optically excited carriers.
Since the leakage photocurrent flows irrespective of signals from the scan lines, even when the TFT is in an OFF state, current flows between the data lines and the pixel electrode. This OFF-state current (OFF current) makes a flicker generated, incurring a poor image quality of the screen of the liquid crystal display device.
Thus, Patent Document 1 (WO 01/067169) proposed a technology of restricting leakage photocurrent from being generated by using a p-type TFT including a full-depletion channel layer.
Further, Patent Document 2 (JP-560-136262A) proposed a technology of restricting leakage photocurrent from being generated by making a semiconductor film of a TFT thinner.
Furthermore, Patent Document 3 (JP-2007-88432A) disclosed a problem in that upon making the semiconductor film of the TFT thinner, the semiconductor film on the bottom of a contact hole is removed by over-etching, so that contact resistance between an interconnection in the hole and a source/drain region increases. To solve this problem, a technology was proposed that the concentration of impurity elements is varied according to a depth of the impurity elements, while the degree of etching (depth) is controlled according to the concentration of the impurity elements.